Add an 'enable_material' GUC.

The logic for determining whether to materialize has been significantly
overhauled for 9.0.  In case there should be any doubt about whether
materialization is a win in any particular case, this should provide a
convenient way of seeing what happens without it; but even with enable_material
turned off, we still materialize in cases where it is required for
correctness.

Thanks to Tom Lane for the review.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.267 2010/04/16 21:46:07 rhaas Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.268 2010/04/19 00:55:25 rhaas Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -2020,6 +2020,22 @@ SET ENABLE_SEQSCAN TO OFF;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-enable-material" xreflabel="enable_material">
+      <term><varname>enable_material</varname> (<type>boolean</type>)</term>
+      <indexterm>
+       <primary><varname>enable_material</> configuration parameter</primary>
+      </indexterm>
+      <listitem>
+       <para>
+        Enables or disables the query planner's use of materialization.
+        It is impossible to suppress materialization entirely,
+        but turning this variable off prevents the planner from inserting
+        materialize nodes except in cases where it is required for correctness.
+        The default is <literal>on</>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-enable-mergejoin" xreflabel="enable_mergejoin">
       <term><varname>enable_mergejoin</varname> (<type>boolean</type>)</term>
       <indexterm>

src/backend/optimizer/path/costsize.c

@@ -59,7 +59,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.216 2010/02/26 02:00:44 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.217 2010/04/19 00:55:25 rhaas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -114,6 +114,7 @@ bool		enable_tidscan = true;
 bool		enable_sort = true;
 bool		enable_hashagg = true;
 bool		enable_nestloop = true;
+bool		enable_material = true;
 bool		enable_mergejoin = true;
 bool		enable_hashjoin = true;
 
@@ -1852,8 +1853,11 @@ cost_mergejoin(MergePath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
 	mat_inner_cost = inner_run_cost +
 		cpu_operator_cost * inner_path_rows * rescanratio;
 
-	/* Prefer materializing if it looks cheaper */
-	if (mat_inner_cost < bare_inner_cost)
+	/*
+	 * Prefer materializing if it looks cheaper, unless the user has asked
+	 * to suppress materialization.
+	 */
+	if (enable_material && mat_inner_cost < bare_inner_cost)
 		path->materialize_inner = true;
 
 	/*
@@ -1867,6 +1871,10 @@ cost_mergejoin(MergePath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
 	 * merge joins can *preserve* the order of their inputs, so they can be
 	 * selected as the input of a mergejoin, and they don't support
 	 * mark/restore at present.
+	 *
+	 * We don't test the value of enable_material here, because materialization
+	 * is required for correctness in this case, and turning it off does not
+	 * entitle us to deliver an invalid plan.
 	 */
 	else if (innersortkeys == NIL &&
 			 !ExecSupportsMarkRestore(inner_path->pathtype))
@@ -1878,8 +1886,11 @@ cost_mergejoin(MergePath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
 	 * pass can be done on-the-fly if it doesn't have to support mark/restore.
 	 * We don't try to adjust the cost estimates for this consideration,
 	 * though.
+	 *
+	 * Since materialization is a performance optimization in this case, rather
+	 * than necessary for correctness, we skip it if enable_material is off.
 	 */
-	else if (innersortkeys != NIL &&
+	else if (enable_material && innersortkeys != NIL &&
 			 relation_byte_size(inner_path_rows, inner_path->parent->width) >
 			 (work_mem * 1024L))
 		path->materialize_inner = true;

src/backend/optimizer/path/joinpath.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/joinpath.c,v 1.132 2010/03/28 22:59:32 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/joinpath.c,v 1.133 2010/04/19 00:55:25 rhaas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -437,10 +437,12 @@ match_unsorted_outer(PlannerInfo *root,
 	else if (nestjoinOK)
 	{
 		/*
-		 * Consider materializing the cheapest inner path, unless it is one
-		 * that materializes its output anyway.
+		 * Consider materializing the cheapest inner path, unless
+		 * enable_material is off or the path in question materializes its
+		 * output anyway.
 		 */
-		if (!ExecMaterializesOutput(inner_cheapest_total->pathtype))
+		if (enable_material &&
+			!ExecMaterializesOutput(inner_cheapest_total->pathtype))
 			matpath = (Path *)
 				create_material_path(innerrel, inner_cheapest_total);
 

src/backend/optimizer/plan/subselect.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.161 2010/02/26 02:00:46 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.162 2010/04/19 00:55:25 rhaas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -578,9 +578,11 @@ build_subplan(PlannerInfo *root, Plan *plan, List *rtable, List *rowmarks,
 		 * is pointless for a direct-correlated subplan, since we'd have to
 		 * recompute its results each time anyway.	For uncorrelated/undirect
 		 * correlated subplans, we add Material unless the subplan's top plan
-		 * node would materialize its output anyway.
+		 * node would materialize its output anyway.  Also, if enable_material
+		 * is false, then the user does not want us to materialize anything
+		 * unnecessarily, so we don't.
 		 */
-		else if (splan->parParam == NIL &&
+		else if (splan->parParam == NIL && enable_material &&
 				 !ExecMaterializesOutput(nodeTag(plan)))
 			plan = materialize_finished_plan(plan);
 

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.547 2010/04/12 09:52:29 heikki Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.548 2010/04/19 00:55:25 rhaas Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -643,6 +643,14 @@ static struct config_bool ConfigureNamesBool[] =
 		&enable_hashagg,
 		true, NULL, NULL
 	},
+	{
+		{"enable_material", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("Enables the planner's use of materialization."),
+			NULL
+		},
+		&enable_material,
+		true, NULL, NULL
+	},
 	{
 		{"enable_nestloop", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of nested-loop join plans."),

src/backend/utils/misc/postgresql.conf.sample

@@ -206,6 +206,7 @@
 #enable_hashagg = on
 #enable_hashjoin = on
 #enable_indexscan = on
+#enable_material = on
 #enable_mergejoin = on
 #enable_nestloop = on
 #enable_seqscan = on

src/include/optimizer/cost.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.100 2010/01/02 16:58:07 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/cost.h,v 1.101 2010/04/19 00:55:26 rhaas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -57,6 +57,7 @@ extern bool enable_tidscan;
 extern bool enable_sort;
 extern bool enable_hashagg;
 extern bool enable_nestloop;
+extern bool enable_material;
 extern bool enable_mergejoin;
 extern bool enable_hashjoin;
 extern int	constraint_exclusion;

src/test/regress/expected/rangefuncs.out

@@ -5,12 +5,13 @@ SELECT name, setting FROM pg_settings WHERE name LIKE 'enable%';
  enable_hashagg    | on
  enable_hashjoin   | on
  enable_indexscan  | on
+ enable_material   | on
  enable_mergejoin  | on
  enable_nestloop   | on
  enable_seqscan    | on
  enable_sort       | on
  enable_tidscan    | on
-(9 rows)
+(10 rows)
 
 CREATE TABLE foo2(fooid int, f2 int);
 INSERT INTO foo2 VALUES(1, 11);